This invention relates to a method and an apparatus for screen printing on thin, curved steel hybrid plates, particularly circuit boards (cards) which are seated in a screen printing nest.
The manufacture of circuit boards from steel hybrid plates, that is, from a plate-like steel substrate which has a thickness of between approximately 0.5 to 6 mm and which is coated with an electrically insulating inorganic material, such as glass, ceramic, enamel or glass-ceramic is known by itself.
It is a frequent occurrence during the firing of the insulating layer or layers that the thin circuit boards undergo a bending deformation. Although this can be counteracted by using a thicker board, such a solution, however, is generally not desirable because of the increased cost and weight of the circuit boards. Further, a thicker metal sheet may cause problems during the firing process.
Further, although it is feasible to print on a curved plate with a curved die, such a printing method is often not sufficiently accurate because a die printing allows layer thicknesses only up to 10 .mu.m whereas significantly greater thicknesses can be achieved with screen printing. Thus, on printed circuit boards often conductor strips having a width in the order of magnitude of 50 .mu.m are printed so that in case of curved plates accuracy problems concerning the contour and the conductor thickness are encountered because the printing screen and the foils for making the printing screen are planar.